The only thing to do in that situation is to compromise one of the communicating parties. If the communicating parties have arranged a safeword to signal they have been compromised, even that technique is useless.
In the case of Snowden and Greenwald, that wasn't going to happen.
It is possible to make storage and communication immune to surveillance. So I ask: should that be illegal?
That is not impervious: compromise an endpoint. It takes work, but in the course of a serious investigation it can get done. At the limit, Van Eck or similar analog-hole analogues.
Anyway, my position is "no, that should not be illegal", though I am not entirely confident in that.
Perhaps, but there are means of communication that are impervious to interception, and that cannot be compromised the way Lavabit might have been.
Should such technologies be outlawed?